1. Field of the Invention
The present invention relates to a decompression braking apparatus for a Diesel engine used as an engine braking apparatus so as to secure a continuous deceleration of a vehicle in which the Diesel engine is mounted during the vehicular run on a long descending slope independently of using a normally used braking apparatus.
2. Description of the Background Art
An engine braking has widely been applied to an industrial vehicle such as an automotive vehicle other than a forced braking through a normally used braking apparatus and a parking braking apparatus.
A Japanese Patent Application First Publication No. Heisei 6-17632 published on Jan. 25, 1994 exemplifies a first previously proposed decompression braking apparatus for a Diesel engine mounted in a vehicle.
A Japanese Utility Model Registration Application First Publication No. Heisei 2-98408 published on Aug. 1, 1990 exemplifies a second previously proposed decompression braking apparatus for the same.
In the second previously proposed decompression braking apparatus disclosed in the latter Japanese document, with a valve stem of one of a pair of exhaust valves of each cylinder of the Diesel engine extended, a stopper arm is positioned so as to interfere with the exhaust valve stem of the corresponding one of the exhaust valves, the stopper arm being projected from a hydraulic pressure cylinder, so that a closed state of the corresponding one of the exhaust valves in a closure stroke thereof is limited to a state immediately before a completely closed state, thus achieving a decompression operation of the Diesel engine.
In the first previously proposed decompression braking apparatus disclosed in the former Japanese document, a special profile of a cam lobe of a cam shaft to control the open and closure of the corresponding exhaust valve is set, a rocker arm is provided in association with the specially profiled cam lobe, and an eccentric bushing member is provided so as to be enabled to displace a swine center of the rocker arm. Then, a lever portion of the eccentric bushing member is pivoted in response to a plunger motion according to an actuation of an associated actuator so as to displace the swing center of the rocker arm, thus a degree of openings in the exhaust valve during the closure stroke is switched so as to achieve the decompression braking.
In the second previously proposed decompression braking apparatus, the degree of openings in the exhaust valve during a closure stroke thereof (a spatial quantity by which the exhaust valve in the closed state is held to be slightly open) is constant irrespective of an engine revolution speed at which a decompression braking request is issued.
In the first previously proposed decompression braking apparatus, on the other hand, the vehicular braking state is limited to two states of the decompression braking apparatus and the normally used braking application. The degree of the openings in the exhaust valve during the closure stroke thereof is not varied according to the engine revolution speed at which the decompression braking request is issued, namely according to whether the engine revolution speed is within a relatively high speed area or a relatively low speed area.
It is desired that the degree of the openings in the exhaust valve during its closure stroke as a requirement that the decompression braking apparatus can achieve is relatively small when the engine revolution speed is relatively low and is relatively large when the engine revolution speed is relatively high in terms of en appropriate decompression braking force. However, a heavy burden is imposed on a valve train of the Diesel engine when the degree of openings in the exhaust valve during its closure stroke is set to the relatively small value with the engine revolution speed in the relatively high speed area.
Therefore, in each of the first and second previously proposed decompression braking apparatus, the degree of the openings in the exhaust valve during its closure stroke cannot help being set to the relatively large value (to a value in the case of the high engine revolution speed).
Consequently, the decompression braking force exhibited during the relatively low engine revolution speed accordingly becomes deficient.